Exterior Insulation, Vapor Permeance and Durability
Understanding how moisture moves through assemblies is one of the fundamental tasks of building science. At our October LAB Event, Jonathan Smegal looked specifically at how water vapor moves through wall assemblies with exterior insulation. Does the vapor permeance of the exterior insulation matter? If so, when and why?
First, Jonathan went over some basics:
- Vapor is not the same as liquid water. Vapor usually becomes a problem when it condenses (becoming liquid water). But before that point, it behaves quite differently from e.g. rain leaks; as a result, it requires different control measures.
- Permeability does not equal permeance. Vapor permeability is a material property. A vapor permeable material will allow some vapor to move through it (this process is called diffusion). Vapor permeance indicates how much vapor can move through a given thickness of a material.
- Airflow typically moves more vapor than diffusion. When we talk about vapor control layers, vapor barriers, etc., we are talking about stopping diffusion. However, air flowing through an assembly will carry vapor with it – in fact, it can move far more water vapor than diffusion.
When the exterior insulation in an assembly has low vapor permeance (e.g. XPS or foil-faced polyisocyanurate), there may be a concern that vapor moving outward from inside the building could condense and become trapped. Vapor will tend to move from the inside out when the indoor environment is warmer and more humid than the outdoors – typical of cold climates.
So what makes vapor condense? Basically it needs to come into contact with a surface that is cold enough (i.e., below the dewpoint). Putting insulation in the stud cavity, i.e., between the exterior sheathing and interior conditioned space, will keep the sheathing temperature closer to the outdoor temperature. Under cold outdoor conditions, more cavity insulation means colder sheathing. Putting insulation on the exterior (outboard of the sheathing) will have the opposite effect. In other words, the ratio of exterior to cavity insulation directly affects sheathing temperature and is therefore critical to avoiding condensation risk inside a wall.
As Jonathan discussed, this ratio can be worked out mathematically (see John Straube’s article on buildingscience.com), and its impact has also been demonstrated through quite a number of studies using measured data (see e.g. this reading list).
If you’re thinking that an interior layer of polyethylene sheeting should stop outward vapor diffusion, you’re right – poly is vapor impermeable (Class I as defined by the International Residential Code). But any deficiencies in the poly installation could result in air leakage, and even small air leaks can carry a lot of vapor. As well, water that gets into the assembly from outside (e.g. a rain leak) will not be able to dry effectively to the interior or the exterior, because you have vapor impermeable materials on both sides. For these reasons, it is worth considering a smart vapor retarder instead of poly if you’re using low vapor permeance exterior insulation in a cold climate and are required to use a vapor barrier on the interior.
Depending on your climate zone and jurisdiction, you may also be able to use kraft paper or painted drywall as your interior vapor control layer. This scenario was tested as part of the Vancouver Test Hut Project, a multi-year, multi-partner project in British Columbia. Jonathan presented results for several sets of test assemblies, including some with low vapor permeance exterior insulation and no interior poly layer. To date, measured data analysis and wall deconstruction have shown that there were no moisture-related durability concerns in the Vancouver, BC, Canada climate when sufficient exterior insulation was installed.
In this and other studies, high vapor permeance exterior insulation did allow faster drying to the exterior following wetting (see Trainor 2014). This result is expected, since water can move out of the assembly through the insulation. On the other hand, high vapor permeance exterior insulation can also allow water vapor to get into the assembly, as happens with inward vapor drive caused by solar heating. Inward vapor drive is particularly a problem in hot-humid climates, but can cause moisture accumulation in cold climates, as discussed in this BEST4 conference paper. Both high and low permeance exterior insulation can be designed to manage inward vapor drives. In the BEST4 study, a design using XPS was found to be effective in preventing inward vapor drives in adhered-veneer assemblies.
We’ll cover inward vapor drives in more detail in a future post. For now, here is Jonathan’s overall conclusion about the vapor permeance of exterior insulation: yes, it matters and should be designed for. But when installed with the correct water management details, in the appropriate thickness, exterior insulation – whether low permeance or high permeance – can increase the durability of the building enclosure.
The post Exterior Insulation, Vapor Permeance and Durability appeared first on RDH Building Science Laboratories.